Intracellular signals that regulate cell growth and division are largely unknown. However, studies in frog eggs and oocytes have identified an intracellular mitotic factor, called maturation promoting factor (MPF), that most likely is common to all eukaryotic cells. MPF has only been partially purified but there is evidence that it is a phosphoprotein of 200-400 k daltons. We have recently shown that MPF can induce a variety of mitotic events in vitro. We have focused primarily on the dissolution of the nuclear envelope and specifically on the increased phosphorylation of the nuclear lamins during mitosis, induced by MPF. Although little is known about the regulation of MPF activity, we have shown that MPF is activated in the oocyte post- translationally, a reaction that we can now carry out in vitro. We propose three general categories of experiments to understand the role of MPF in cell cycle regulation. First we will attempt to purify MPF or its precursor from Xenopus oocytes. We also plan to examine MPF in yeast by determining whether any known cdc mutants or other ts lethals are deficient in MPF, or by purifying yeast MPF directly. Second we hope to reconstruct the enzymatic processes that lie between MPF and lamin phosphorylation. Specifically by a detailed study of phosphorylation sites we will try to determine if there is a new mitosis specific lamin kinase. If so we will try to determine how its activity is regulated by MPF. Finally we will investigate the role of protein synthesis in the Xenopus cell cycle and evaluate whether proteins that are periodically degraded at each division, called "cyclins" are involved in regulating MPF and the mitotic state.